A knife is a broad word encompassing a large number of tools. The cheapest knife to buy and maintain is an x-acto knife.
The no. 1 knife handle with the no. 11 blade is the most common knife we associate with X-Acto but they make a broad product line for model makers. The blades are stainless and never get as sharp as a steel blade.
A more comfortable handle is the X-2000. X-Acto knife blades are cheap enough that sharpening is not required. The blades are simply replaced.
A better knife is a violin makers knife. You can sharpen the blade and trim back the handle for a lifetime. They are available in German steel and Japanese Steel. The German steel is more durable (chips less easily). The Japanese steel gets sharper and stays sharp longer but chips quite easily.
Sharpening knives is a task of patience. You must completely sharpen one face to completion before moving on to a finer grit. You may stop at anytime the knife is sharp but what is described below will make a knife sharp enough to shave.
The angle you sharpen at is chosen based on what the knife will be used for. Use a blunt angle for rough work and a durable blade and use a sharp angle for delicate work.
- Rough out both sides in a single plane with a course stone.
- Polish the faces with a fine stone.
- Hone the knife on a leather strop with red compound (rouge).
- Hone the knife on a 2nd leather strop with white compound.
- Do not switch to fine grit until all chips have been removed.
- Be careful to not round the edge. You need one plane from the shank of the knife to the edge.
- Sharpen both sides evenly.
- Do not pull a knife towards your body or fingers.